It is well-known to eliminate this type of pollutants by passing the exhaust gas through catalysts (referred to as DeNO.sub.x catalysts) intended for nitrogen oxides conversion. The known catalysts being active within a given temperature range, several catalysts having different formulations, i.e. different activity ranges, may be placed in the catalytic muffler. The sphere of action of the catalytic elements is thus extended. However, in this context, a problem comes up when the exhaust gas is not, at the level of the catalyst(s), within a temperature range for which conversion of the nitrogen oxides is sufficient.
More precisely, a problem comes up when a first catalyst is active in a first temperature range and a second catalyst is active in a second temperature range, and when these ranges are such that there is a range of temperatures between and above the two temperature ranges defined above in which the nitrogen oxides conversion will be low.
Furthermore, at the lowest temperatures, i.e. before the gas reaches a temperature where it is likely to be converted by one of the catalysts, there is also a lack of nitrogen oxides conversion.
By way of illustration, the formulations used for low temperatures are of the Platinum/Alumina or Platinum/Zeolite type. The temperatures for which these catalysts are the most active are from 200.degree. C. to 250.degree. C.
Catalysts referred to as "High-temperature" catalysts are generally active between 300.degree. C. and 500.degree. C. These are for example Copper/Zeolite type catalysts.
It is of course obvious that, between these two ranges, i.e. here between 250.degree. C. and 300.degree. C., no nitrogen oxides conversion can be performed effectively. Below 200.degree. C. and above 500.degree. C., the problem is the same.
There are also catalysts on which the nitrogen oxides are adsorbed as nitrates when the temperature of the gas flowing therethrough is in a certain range. These catalysts are commonly referred to as "NO.sub.x traps". The nitrates thus adsorbed can thereafter be removed either thermally or through a momentary combustion with an air/fuel ratio above 1. The latter action is well-suited for exhaust lines of lean-burn spark-ignition engines.
Patent application EP-A1-0,540,280 describes a system comprising a NO.sub.x trap, provided with a gas reheating system, followed by a nitrogen oxides reduction catalyst. Both catalysts are mounted on a line bypassing the main exhaust line. According to this document, a valve system allows to decrease the GHSV (ratio of the flow of gas to the volume of catalyst showing the contact time between the gas and the catalyst) during trap emptying phases. The conversion coefficient of the NO.sub.x on the nitrogen oxides reduction catalyst is thus improved. However, with this configuration, the part of the gaseous stream passing through the main line does not flow through the NO.sub.x reduction catalyst. Furthermore, there may be temperature ranges where the NO.sub.x trap and the NO.sub.x reduction catalyst are active, in which case the NO.sub.x will be stored in the trap whereas they might have been reduced. According to this prior art, the flow regulation is not intended to optimize the amount of gas to be reheated for the thermal elimination of the NO.sub.x previously stored, but it is rather intended to improve conversion of the nitrogen oxides on the nitrogen oxides reduction catalyst.
The aforementioned problem relative to the low conversion in certain temperature ranges, as well as others, can be solved according to the invention.